Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.1.4Radioactivity

Isotopes — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 5 of 20

📚 Edexcel GCSE Combined Science 1SC0 - Physics Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 8What isotopes are

  • Definition
    • isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
  • Naming
    • written as the symbol or name, a dash and the mass number, such as C-14 or carbon-14.
    • Carbon-14
      • 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
    • Written form
      • it can also be shown with the mass number as a superscript before the symbol.
    • Hydrogen
      • has three isotopes that differ only in neutron number.

Part 2 of 8Chemical and physical properties of isotopes

  • Same chemistry
    • isotopes have the same number of outer electrons and so the same electronic configuration, which determines chemical behaviour.
  • Neutrons
    • they are neutral and add mass only.
    • Appearance
      • a sample of C-14 looks identical to C-12.
  • Different physical properties
    • extra mass affects density, melting point and boiling point.
  • Heavy water
    • deuterium oxide has relative formula mass 20 compared with 18 for water, so it is about 20% heavier yet looks, tastes and feels like water.
    • Hazard
      • it should not be drunk because it interferes with biochemical reactions in cells.

Part 3 of 8Relative atomic mass

  • Symbol
    • Ar, found beneath the symbol in the Periodic Table, with the atomic number above it.
  • Standard
    • carbon-12 is the reference atom, with a fixed mass of 12 units.
  • Definition
    • Ar is the average mass of the isotopes of an element compared with 1/12 of the mass of a carbon-12 atom.
    • Examples
      • carbon is 12, magnesium is 24 , hydrogen is 1 (one-twelfth of carbon).
        • (twice as heavy as carbon)

Part 4 of 8Calculating Ar from isotopes

  1. Multiply each isotope percentage by its mass number.
  2. Add all the products together.
  3. Divide the total by 100.
  • Equation
    • Ar = [ + (% isotope 2 × mass 2) + …] / 100.
      • (% isotope 1 × mass 1)
  • Extending
    • add more terms when there are more isotopes.
    • Rubidium example
      • / 100 = 85.6.
        • (72 × 85 + 28 × 87)

Part 5 of 8Mass number compared with relative atomic mass

  • Difference
    • mass number is a whole number for one isotope, while Ar is a weighted average over all isotopes.
  • Lithium
    • its Ar is 6.94, shown as 7 when rounded to a whole number.
    • Why it looks the same
      • Periodic Tables often show Ar rounded, so it appears equal to a mass number.
  • Chlorine
    • its Ar is 35.5 because of the mixture of Cl-35 and Cl-37, so it is the exception where Ar is not used as the mass number.

Part 6 of 8Isotopes of hydrogen and carbon

IsotopeProtonsNeutronsElectrons
Hydrogen-1101
Hydrogen-2 (deuterium)111
Hydrogen-3 (tritium)121
Carbon-12666
Carbon-14686
  • Reading the table
    • the proton and electron numbers never change within an element.
  • What changes
    • only the neutron number, and therefore the mass number.

Part 7 of 8Steps for exam questions on isotopes

  1. Identify the proton number from the symbol or the Periodic Table.
  2. Subtract the proton number from the mass number to find neutrons.
  3. State that the electrons equal the protons in a neutral atom.
  4. For chemical behaviour, refer to the identical electronic configuration.
  5. For physical behaviour, refer to the different mass.
  • Typical statement
    • isotopes have the same chemical properties because they have the same number of electrons in the outer shell.
  • Typical statement 2
    • their densities and boiling points differ because their masses differ.

Part 8 of 8Worked examples

  1. Chlorine has two isotopes: 75% Cl-35 and 25% Cl-37. Calculate Ar. [2]2 marks

  2. State the number of protons, neutrons and electrons in carbon-14 and in carbon-12. [3]3 marks

  3. Explain why two isotopes of an element have the same chemical properties. [2]2 marks

  4. Magnesium has isotopes Mg-24 (79%), Mg-25 (10%) and Mg-26 (11%). Calculate Ar. [2]2 marks

Key terms

TermMeaning
IsotopeAtom of the same element with the same protons but a different number of neutrons.
Relative atomic mass (Ar)Average mass of an element isotopes compared with 1/12 of a carbon-12 atom.
Carbon-12The standard atom, with a fixed mass of 12 units.
AbundanceThe percentage of each isotope in a natural sample.
Heavy waterWater made with deuterium, relative formula mass 20.

Avoid these mistakes

  1. Saying isotopes have different numbers of electrons — they differ only in neutrons.
  2. Thinking isotopes react differently — their chemistry is the same.
  3. Calculating Ar by averaging the mass numbers without weighting — use the percentages.
  4. Treating mass number and Ar as identical — Ar is an average over isotopes.

Last updated: